Low power consumption is desirable for any LCD—especially one installed in a mobile device such as a notebook computer or smart phone. In these devices, power consumption in the LCD significantly affects battery life.
In a conventional LCD array, each column of pixels is accessed through a data line. The capacitance of the data line is repeatedly charged and discharged at a frequency of Nselect times the frame frequency, where Nselect is the number of rows of the array. A frame frequency of 60 Hertz (Hz) is conventionally used to display video. For static images, however, a much lower frame frequency would suffice, the lower frame frequency demanding less frequent capacitive charging and therefore consuming less power.
To operate a conventional pixel array at a reduced frame frequency, however, the leakage current through each pixel and associated thin-film transistor (TFT) must be suppressed so that the image is retained over a longer period between successive refresh events. It has been demonstrated that obliquely evaporated SiOx liquid-crystal alignment layers, having increased ionic impurity adsorption, may improve voltage holding in an LCD array. This feature may enable the pixels of the display to operate at refresh frequencies as low as 1 Hz. TFT Ioff may be reduced to less than 10 femtoamperes (fA) in a recently developed, optimized oxide TFT. In addition, a TFT engineered with an especially narrow channel (low in width-to-length ratio) may exhibit attractively low Ioff.
To display video, however, Ion must remain high enough to charge the pixels at the conventional frame frequency (e.g., 60 Hz). All told, this would require an Ion/Ioff ratio of at least eight orders of magnitude. In a manufacturing environment, such a ratio may be difficult to achieve, especially when photo-leakage current in the TFT is taken into account.